This invention relates to a manufacturing method of brazing uniform plate/plate and plate/fin multi-channeled structures using an amorphous brazing foil as a brazing filler metal.
Brazing is a process for joining metal parts, often of dissimilar composition, to each other. Typically, a brazing filler metal that has a melting point lower than that of the parts to be joined is interposed between the parts to form an assembly. The assembly is then heated to a temperature sufficient to melt the brazing filler metal. Upon cooling, a strong and preferably corrosion resistant joint is formed.
One class of products produced by brazing processes is three-dimensional structures comprised of a number of alternating metal flat plates and fins or corrugated plates kept in tight, physical and sealed contact. This contact occurs through joints formed across multiple local areas positioned between flat plates and fins, as in the case of plate/fin heat exchangers or between corrugations stamped out in special patterns in the plate/plate case. The corrugation profiles may have a chevron pattern or pressed-out indentations of various circular forms or some other profiles. In the brazed state these indentations are joined with flat plates or with each other forming an elaborate system of channels or interlocking cavities. Eventually, in service, one hot and one cool liquid and/or gas media flow separately in these channels exchanging heat and thus saving energy. In most cases these structures are made from heat and corrosion resistant steels and alloys as base metals and operate at high temperatures as coolers in utility hot water systems and as heat exchangers and recuperators in aerospace, chemical, food and other process industries.
The most effective physical contact of the initial gaps existing between some area of the plate cross-section is made with brazing carried out using a preplaced filler metal preform between base metal parts. This preform may be in a powder or a foil form.
The majority of Ni- and Co-based advanced filler metals that can be used for joining these structures contain a substantial amount of metalloid elements such as boron, silicon and/or phosphorus. Consequently, such alloys are very brittle in conventional crystalline form and available only as powders, powder-binder pastes and tapes and bulky cast preforms. Powders and powder-based preforms do not easily permit brazing of complex forms. However, these Ni- and Co-based alloys can be transformed into a ductile, flexible foil that is produced utilizing rapid solidification technology and which has an amorphous structure in the solid state. Such amorphous alloys for brazing applications are disclosed in many patents, for example U.S. Pat. Nos. 4,148,973 and 4,745,037 (NDC, 1979 1988). In spite of substantial advantages of rapid solidification technology achieved so far, the foil thus produced has cross-sectional and longitudinal thickness variations, sometimes exceeding xc2x140%.
So far, practically all multi-channeled brazed structures have been produced using filler metal powder sprayed on the base metal parts and/or amorphous brazing foil. The use of powder filler metals requires an excessive amount of material per product cross-section and, most importantly, results in uneven, porous and poor quality joints. The use of foil in constrained assemblies, while being much more effective than powder, necessitates small variations in the foil thickness. This is particularly important when these assembled alternating base metal plates and foil preforms are constrained from mutual movement during brazing. It is rather difficult to satisfy the limits of these variations when using amorphous foil in constrained assemblies to be brazed because the foil has large local thickness variations, up to xc2x150%, due to specifics of the rapid solidification technology. Further, to optimize the brazed structure performance when choosing the proper average foil thickness relative to the base metal plate thickness and geometry, one needs to carry out tedious preliminary experiments with foils having different thicknesses. Moreover, nonadjustable, constrained assemblies require that all parts have very precise dimensions and a very accurate part placement that is difficult and expensive to satisfy using the existing technology. To illustrate, generally the height of channels, and therefore the height of fins or elevations in corrugated parts, is mostly within 3 to 10 mm, whereas the width and the length of the plates and fins are in the range of hundreds of millimeters. Concurrently, the gaps between parts to be joined should be only about 25 to a maximum of about 75 xcexcm. Any potential variations in plate dimensions or local defects such as indents, etc., that are larger than xc2x110 xcexcm (i.e., only about 0.1 to 0.3% of the plate height) may result in unbrazed and, consequently, defective areas. The only way to seal these local large gaps is to fill them with a filler metal. In contrast, when the gaps are large, but the amount of available filler metal is small or the filler metal has poor flow as powder does, then filling of these excessive gaps may not be sufficient in a mechanically constrained assembly of plates and preforms. As a result, there may be large unbrazed areas.
There is another major objective to be addressed when producing three-dimensional structures: namely, the pressure of the operating media used in the structures or changes thereof during long term service that these structures should withstand. This pressure should be safely contained by the total strength of the structure, which is determined as the product of the total joint cross-section of all contact areas times the joint strength. Whereas the joint strength is a parameter determined mostly by the joint microstructure that, in turn, is affected by the time-temperature brazing conditions, it is rather difficult to predict and, even more, difficult to regulate the joint cross-section in each contact area. The same is true for the thickness of each joint when these structures are manufactured using foils with varied thicknesses and fins and parts with varied dimensions. In the ideal case of a high strength joint, a potential failure of the brazed structure under the critical internal pressure would occur in the structural parts made of the base metal.
Thus, there is a continuing need for an improved method of brazing complex three-dimensional plate/plate and plate/fin structures that can provide strong joints with controlled cross-section dimensions without being overly dependent on: (a) brazing foil thickness and its variations; and (b) the shape and accuracy of dimensions of fins and profiles.
Meanwhile, there exists experimental data showing the positive effect of load applied to specimens having gaps that vary with the load and which are brazed using amorphous foil. This data indicates the importance of load in the improvement of liquid filler metal wetting of rough gap surfaces and formation of non-porous brazes. Moreover, the self-adjusting interplay between the surface tension of a liquid filler metal and the applied load also optimizes the thickness, the microstructure and, most importantly, the strength of brazes. This fundamental load effect provides the scientific basis for the proposed method of the present invention to improve brazed multi-channeled structures.
The above-noted problems, and others, are overcome by this invention, in which the present brazing methods are improved by using preplaced amorphous or brazing foil and assembling an unconstrained and properly and uniformly loaded stack of alternating sets of plate/foil preform/plate or plate/foil preform/fin/foil preform/plate.
This improvement is embodied in a brazing method comprising the steps of interposing an interlayer in an amorphous foil form between plates and fins to be joined, assembling parts in an unconstrained stack, applying a controlled load on the top of the stack, heating the assembly under suitable conditions to a temperature at which the interlayer melts and reacts with the base metal parts, and cooling the assembly to produce a structure with even-sized joints having optimal dimensions, shape and high strength.
The invention also comprises a brazed structure produced by the method described hereinabove.